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It occurs to me that a lot of power is supplied to pump water into my home -- into my showers, dishwasher, sinks, toilets, etc. And then, as that water plummets from the second story bathroom back into the ground, it builds up a lot of power due to gravity -- and it just goes whooshing away. So let's develop some sort of mechanism that will harvest the energy of that moving water as it leaves our homes. The initial thought is turbines or wheels, sure, but you have to remember the stuff that is leaving the house in that water: hair, food scraps, and all your bathroom functions. So we need to either develop a system that cannot be jammed, filter & divert the water before it reaches our device, or develop a system that harvests the energy without interfering with the flow.

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The ubiquitous PKM back-of-the-envelope

Water leaving your house isn't under significant pressure so will only contribute its gravitational potential energy.

GPE = mass * gravity * change in height
1 litre of water conveniently has a mass of 1kg so has 9.8J/m of energy.

1kwh is 3600000 joules, so would require 3600000 litre-metres of water. I'll assume that you are harvesting the energy from the drop between your bathroom sink, elevation perhaps 5 metres above the ground floor) so this would require 3600000 / 5 = 720,000 litres per kWh. For our friends in Liberia this is over 190,000 US gallons, or approximately 3,600 bathtubs full.

Assuming the average person uses 200 litres per day (liberal estimate, in the UK that figure is closer to 150) that's about ten years worth of water usage for one kWh of electricity.

tl;dr- not worth it.

LOL, math FTW.

I saw something on this old house where they had used city water to power something in an emergancy. it ran directly off the main water line and you basically had to waste a bunch of water to power something but it was only for an emergancy. I cant remember the name of the product though

It was a sump pump. In an emergency, your power goes out, but water pressure almost never goes down. It makes a venturi to act as a secondary sump pump (to prevent basement flooding) - and it only kicks in when the primary sump fails.

. It would be much easier to put your generator on the incoming water line (clean water and more pressure), but even if you do that, you're not going to have enough flow to generate much electricity. . There's another topic on this subject, but I can't find it right now. Someone actually did this (on the incoming line) and, IIRC, was able to power an LED for a few seconds.

I figure household pressure at 20 or so psi during a long wasteful shower could...do some neat stuff...charge a phone... not a tonne though, you're right.

I calced it once, I think I can up with 2 kwh per month, not worth the effort.

Wow, that is lower than I expected. I didn't foresee anything marvelous, but I had hoped for more than that. How did you run those numbers?

The problem with tapping the incoming line is that it would cause a pressure drop in the water in the house. This, however, ought to pull energy from the water after it has been pumped around and is on its way out.

It would seem to me that there is no way to harvest energy from water flow without interfering with the flow. You would be getting energy from the momentum built up in the water so in order to get that energy you have to interfere with the flow of the water. No way around it.

True, I should clarify. You would naturally slow the flow, but it would need to be done in a non-obstructive manner. I.e., no paddles or blades or other typical methods. You could, hypothetically, use the power of the water to continuously alter the shape of the pipe and draw power from there.